Pumps used in infusion therapy typically have pressure sensing mechanisms and respond in some fashion to pressure levels indicative of a fault condition. For example, a sudden rise in the pressure of the fluid in the IV delivery tubing may be indicative of some occlusion in the line between the pump and the patient. In such a case, the infusion pump stops pumping fluid and provides an audible and visual alarm to alert a human operator to the presence of the fault.
It is also desirable for the pump to react in two further ways. First, the pump should close the fluid pathway until the human operator takes positive steps to set the condition right. The rationale for this reaction is that, if the occlusion should clear spontaneously, there would be an open flow path, possibly allowing gravity flow in excess of the planned regimen. Second, the pump should relieve pressure in the IV line between the pump and the occlusion to prevent delivering a sudden bolus of pressurized fluid to the patient when the occlusion is cleared.
These two reactions are accomplished in the infusion pump sold under the trade designation "Model 480", for example, by Minnesota Mining and Manufacturing Company, St. Paul, Minn. This infusion pump is used with a tubing assembly having a cassette with two pumping chambers generally as disclosed in coassigned U.S. Pat. Nos. 4,236,880; 4,277,226; 4,322,201; 4,382,753; 4,391,600; and 4,410,322, which are hereby incorporated by reference. When an occlusion fault condition is detected, the motor is reversed so as to relieve pressure between the pump and the occlusion site.
FIGS. 1 and 2 illustrate various details of the "Model 480" infusion pump. The "Model 480" infusion pump has a mechanism for closing the IV tubing to fluid flow as the pump motor 25 is reversed. That mechanism includes a wheel 20, having a ratcheted or knurled circumference 22, mounted on the drive shaft 24 of the motor 25. A generally Z-shaped spring 26 is mounted in the infusion pump, with one end of the "Z-spring" 26 continually sliding past the ratcheted circumference 22 of the wheel 20 when the drive shaft 24 is rotating in the normal, forward direction (clockwise in FIG. 2). When the drive shaft 24 is rotating in the normal, forward direction, the infusion pump is pumping IV fluid in the downstream direction to regulate the flow of the fluid.
When the pump motor 25 reverses the direction of rotation of the wheel 20 (now counterclockwise in FIG. 2) to relieve pressure in the IV tubing 27 downstream of the infusion pump, the Z-spring 26 catches the ratcheted circumference 22 of the wheel 20. And, when the Z-spring 26 catches the circumference 22, the Z-spring 26 bends and applies pressure to a valve stem 28 to close a ribbed "overpressure" valve 30, thereby occluding the lumen of the IV tubing set 32. In practice, the Z-spring 26 allows the drive shaft 24 to back up about one eighth of a turn before closing the valve 30.
The valve 30 in the "Model 480" infusion pump serves an additional purpose of occluding flow through the lumen of the IV tubing until pressure upstream of the valve 30 builds up to a predetermined amount. See, in particular, coassigned U.S. Pat. No. 4,322,201. An overpressure piston 34 is provided with a spring-biasing mechanism 36 to hold the overpressure valve 30 in its closed position until a predetermined cracking pressure is exerted by an overpressure chamber 38 of the pumping cassette 40 against the overpressure piston 34. During normal operation of the infusion pump, once this predetermined cracking pressure is obtained, the overpressure piston 34 allows a pin 42 mounted on the overpressure valve 30 to move along a longitudinally-extending slot 44 in the piston 34 to permit the overpressure valve 30 to open or close.
The overpressure piston 34 also includes a mechanism to trigger an alarm or signal in response to high fluid pressure in the overpressure chamber 38. That mechanism comprises a circular contact plate 55 mounted on the piston, which will engage one or both of two telescoping, spring-loaded, overpressure contact probes 57 or 58. Contact probes 57 and 58 are positioned and adapted to provide an alarm signal indicating high fluid pressure in the overpressure chamber 38, which may indicate that the IV line downstream of the infusion pump is occluded. A microprocessor (not shown) receives the signal indicating an overpressure condition, and reverses the motor 25 until the overpressure condition is relieved, at which point the infusion pump stops and enters the standby mode.
First and second pumping pistons 46 and 48 and first and second valves 50 and 52 are also provided in the "Model 480" infusion pump, and are also described in U.S. Pat. No. 4,322,201. The pistons 46 and 48 act on pumping chambers 54 and 56 to pump fluid through the cassette 40.
While the Z-spring mechanism has been successful in use, it has been difficult to manufacture and assemble in the infusion pump. In order to obtain consistent results with the spring, much time and care must be taken by skilled production workers.